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Abstract

We introduce a compact submicron structure consisting of multiple optical microcavities at both the entrance and exit sides of a subwavelength plasmonic slit filled with an absorbing material. We show that such microcavity structures at the entrance side of the slit can greatly enhance the coupling of the incident light into the slit, by improving the impedance matching between the incident plane wave and the slit mode. In addition, the microcavity structures can also increase the reflectivities at both sides of the slit, and therefore the resonant field enhancement. Thus, such structures can greatly enhance the absorption cross section of the slit. An optimized submicron structure consisting of two microcavities at each of the entrance and exit sides of the slit leads to ~9.3 times absorption enhancement at the optical communication wavelength compared to an optimized slit without microcavities.

Figures (5)

(a) Schematic of a structure consisting of a slit in a silver film with N microcavities at the entrance side, and M microcavities at the exit side of the slit deposited on a silica substrate. The slit is filled with germanium, while the microcavities are filled with silica. (b) Schematic of a bulk germanium photodetector with an anti-reflection coating. (c) Schematic defining the transmission cross section σT of a silver-germanium-silver waveguide through the structure above the entrance side of the slit of Fig. 1(a) for a normally incident plane wave from air. (d) Schematic defining the reflection coefficient r1 of the fundamental TM mode of a silver-germanium-silver waveguide at the interface of such a waveguide with the structure above the entrance side of the slit of Fig. 1(a). (e) Schematic defining the reflection coefficient r2 of the fundamental TM mode of a silver-germanium-silver waveguide at the interface of such a waveguide with the structure below the exit side of the slit of Fig. 1(a).

(a) Schematic of a structure consisting of a single slit in a silver film deposited on a silica substrate. The slit is filled with germanium. (b) Absorption cross section σA in units of w (black line and circles), and absorption enhancement factor η (red line and circles) for the structure of Fig. 2(a) as a function of slit length L calculated using FDFD (circles) and scattering matrix theory (solid line). Results are shown for w = 50nm and λ0 = 1.55μm.

(a) Schematic of a structure consisting of a slit in a silver film deposited on a silica substrate with a single microcavity at the entrance side of the slit. The slit is filled with germanium, while the microcavity is filled with silica. (b) Absorption cross section σA in units of w for the structure of Fig. 3(a) as a function of width wT1 and length dT1 of the microcavity calculated using FDFD. Results are shown for w = 50nm, L = 122nm, and λ0 = 1.55μm. (c) Profile of the magnetic field amplitude enhancement with respect to the field amplitude of the incident plane wave for the structure of Fig. 3(a) for wT1 = 380nm and dT1 = 200nm. All other parameters are as in Fig. 3(b).

(a) Profile of the magnetic field amplitude enhancement with respect to the field amplitude of the incident plane wave for the optimized structure of Fig. 1(a) with N = M = 1. Results are shown for (wT1, dT1, wB1, dB1) = (300, 260, 860, 380) nm. All other parameters are as in Fig. 3(b). (b) Profile of the magnetic field amplitude enhancement with respect to the field amplitude of the incident plane wave for the optimized structure of Fig. 1(a) with N = M = 2. Results are shown for (wT1, dT1, wT2, dT2, wB1, dB1, wB2, dB2) = (1000, 390, 540, 200, 660, 220, 180, 210) nm. All other parameters are as in Fig. 3(b).